Brushless dc drive

ABSTRACT

In a brushless direct-current drive with a synchronous motor ( 10 ), which has a stator ( 12 ) that supports a multi-phase stator winding ( 15 ) and a rotor ( 13 ) equipped with permanent magnet poles ( 20 ), and with a switch unit ( 11 ), which precedes the stator winding ( 15 ), for commutating the stator winding ( 15 ), in order to produce a fail-silent behavior, a field excitation winding ( 21 ) is disposed in the rotor ( 13 ), which winding can be supplied with current in the event of a malfunction so that it generates a magnetic flux oriented in the opposite direction from the magnetic flux of the permanent magnet poles ( 20 ) (FIG.  2 ).

PRIOR ART

[0001] The invention is based on a brushless direct-current driveaccording to the preamble to claim 1.

[0002] In motor vehicles, permanent magnet-excited, brushlessdirect-current drives are used for a variety of purposes, including forelectric steering boosters. These direct-current drives have asynchronous motor with a stator or armature winding and a permanentmagnet-excited rotor. The armature winding is connected to the DCvoltage network by means of an inverter in a bridge circuit withsemiconductor power switches. The inverter that executes the commutationof the stator winding is triggered by an electronic control unit.

[0003] DE 37 09 168 A1 describes a synchronous motor operated in a DCvoltage network in which three semiconductor power switches triggered bya control unit are disposed in the winding strands of the statorwinding, which is wired in a star pattern. If malfunctions occur in thestator winding and/or in the power switches, then the direct-currentdrive can generate a strong electromagnetic braking moment without theapplication of a DC voltage since the synchronous motor then functionsas a generator in opposition to a low-ohm load resistance. In manyapplications, such a braking moment impairs the function of the unit orsystem in which the direct-current drive is being used. Thus for examplein electric steering boosters, the braking moment occurring in the eventof a malfunction exerts considerable steering forces, which the drivermust resist with his own physical strength in order to take thenecessary countermeasures. There is thus the danger that the driver willno longer be able to steer the vehicle as desired and will lose controlof the vehicle. It is known to provide these direct-current drives withdevices, which in the event of such a malfunction, produce a so-calledfail-silent behavior of the direct-current drive, i.e. themalfunctioning direct-current drive has no disruptive or disadvantageousinfluence on the unit or the system and the system therefore functionsas if the direct-current drive were not present.

[0004] In a known electric steering booster, the desired fail-silentbehavior is produced by means of a mechanical clutch via which thedriven shaft of the synchronous motor engages the steering booster. Inthe event of a malfunction, the clutch disengages and consequentlymechanically disconnects the motor from the steering system.

[0005] A hybrid-excited electrical machine is known (EP 0 729 217 B1),in which the magnetic field of the rotor is generated both by means ofpermanent magnets and by means of a field excitation winding, which issupplied with excitation current via rotor slip rings. The rotor isaxially divided into two rotor halves, which are mounted spaced axiallyapart from each other on the rotor shaft. Receiving openings areprovided in the lamination bundle of each rotor half and the permanentmagnets are inserted into them. In terms of their polarity, thepermanent magnets are disposed in the rotor halves so that in the onerotor half, they point toward the air gap of the machine with theirnorth pole and in the other rotor half, they point toward the air gap ofthe machine with their south pole. The permanent magnets in the tworotor halves are offset in relation to one another by a pole division.The field excitation winding embodied in the form of an annular coil isinserted into the intermediary space between the two rotor halves. Ifthe field excitation winding is supplied with DC voltage, then amagnetic flux is generated, which intensifies or weakens the magneticflux of the permanent magnets depending on the flow direction of theexcitation current. This yields a large regulating range for the speedand the voltage of the machine.

ADVANTAGES OF THE INVENTION

[0006] The brushless direct-current drive according to the invention,with the features of claim 1 has the advantage that the desiredfail-silent behavior of the direct-current drive is achieved throughsimple control actions in the direct-current drive itself, withoutcostly external components of the kind represented by mechanicalclutches. Through appropriate triggering of the field excitationwinding, in the event of a malfunction, the magnetic field of thepermanent magnets can be weakened or strengthened by means of themagnetic field of the field excitation winding so that no inducedvoltage or only a reduced one occurs in the synchronous motor andconsequently, no short circuit current or only a reduced one can flow,which generates no braking moment or only a very slight one. In thisconnection, a moderate temperature-induced decrease in the magneticfield leads to a decrease in the braking moment and prevents anirreversible demagnetization of the permanent magnets and thereforeprevents them from being permanently damaged. If a reduction in thebraking moment is insufficient, then the magnetic field as a whole canbe reduced to zero, but permanent damage to the permanent magnets mustthen be accepted.

[0007] If the field excitation winding of the rotor is supplied withcurrent not only in the event of a malfunction, but also during normaloperation, then on the one hand, an addition of the magnetic fields inthe motor achieves a high power density and on the other hand inspeed-variable operation, higher speeds can be achieved through adeliberately executed field weakening.

[0008] Advantageous modifications and improvements of the direct-currentdrive disclosed in claim 1 are possible by means of the steps taken inthe remaining claims.

[0009] According to a preferred embodiment of the invention, the fieldexcitation winding has a number of coils that corresponds to the numberof permanent magnets and each coil is wound around one of the permanentmagnets. The coils are connected in parallel or in series and areconnected to a pair of rotor slip rings that are connected to the rotorshaft in a non-rotating fashion. This structural integration of thefield excitation winding into the synchronous motor requires only aslight additional structural and production-related expense in order toproduce the desired fail-silent behavior.

DRAWINGS

[0010] The invention will be explained in detail in the followingdescription, in conjunction with an exemplary embodiment shown in thedrawings.

[0011]FIG. 1 shows a circuit diagram of a brushless direct-currentdrive,

[0012]FIG. 2 is a schematic cross section through a three-phase,four-poled synchronous motor in the direct-current drive according toFIG. 1,

[0013]FIG. 3 is a schematic view of the rotor of the synchronous motorin FIG. 2.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0014] The brushless direct-current drive shown in the circuit diagramin FIG. 1 has a synchronous motor 10, which is operated by means of aswitch unit 11 for electronic commutation in a DC voltage network, whichis characterized in FIG. 1 with “+” and “−” and is connected toconnecting terminals 30, 31 of the direct-current drive.

[0015] In a known fashion, the synchronous motor 10 has a stator 12 anda rotor 13, which is supported on a rotor shaft 14 (FIG. 2), which is inturn supported in rotary fashion in a housing. The stator 12 supports anarmature or stator winding 15, which is embodied as three-phase in theexemplary embodiment and whose winding phases 151-153 are wired in astar pattern. The winding connections 1, 2, and 3 of the stator winding15 are each connected to the switch unit 11 via a connecting line 16.

[0016] The switch unit 11, which is embodied as a B6 inverter, has sixsemiconductor power switches 15 in a bridge circuit, which in theexemplary embodiment are embodied as MOS-FETs. The connecting lines 16leading to the winding connections 1, 2, and 3 are each connected to oneof the taps 4, 5, and 6 of bridge branches, which are each comprised oftwo power switches 17 connected in series, and are disposed in theconnection between each pair of power switches 17. In order to commutatethe stator winding 15, i.e. in order to connect the winding phases151-153 to the DC voltage network at the correct times in coordinationwith the rotation position of the rotor 13, the power switches 17 can betriggered by an electronic control unit 18.

[0017] The rotor 13, which is depicted only symbolically in FIG. 1 andis depicted in a schematic sectional view in FIGS. 2 and 3, has a rotorbody 19, which is comprised of a lamination bundle or a solid ironbundle and which is supported on the rotor shaft 14 in a non-rotatingfashion, and a number of permanent magnet poles 20 affixed to theoutside of the rotor body 19. The rotor 13 shown in FIG. 2 is embodiedas four-poled and consequently has four permanent magnet poles 20, whichare disposed offset by 90° from one another on the circumference of therotor body 19, wherein succeeding permanent magnet poles 20 havealternating polarities “N” and “S”, as shown in FIG. 3. The rotor body19 also accommodates a field excitation winding 21, which is connectedto a pair of rotor slip rings 22, 23 that are mounted on the rotor shaft14 in a non-rotating fashion. The field excitation winding 21 iscomprised of a total of four coils 24, wherein each coil 24 is woundaround one of the permanent magnet poles 20, i.e. the permanent magnetpole 20 is wound around all four of its side surfaces that extend in theaxial direction and in the circumference direction. In the exemplaryembodiment, the coils 24 are connected in parallel and the parallelcircuit is connected to the rotor slip rings 22, 23. Alternatively, thecoils 24 can also be connected in series. The coils 24 are wound so thatthe excitation current taken from the rotor slip rings 22, 23 producesan inverse magnetic field in succeeding coils 24.

[0018] The field excitation winding 21 is part of a device intended toproduce a so-called fail-silent behavior of the direct-current drive,which assures that in the event of a malfunction in the direct-currentdrive, which can be caused by a defective power switch 17 or a windingshort circuit in the stator winding 15 for example, the systemcooperating with the direct-current drive is not disadvantageouslyinfluenced or impaired. In addition to the field excitation winding 21,this device also has a controller 25 integrated into the control unit18, three instrument shunts 26 each disposed in a respective windingphase 151, 152, 153, and a temperature sensor 27 that detects the motortemperature. The instrument shunts 26 and the temperature sensor 27 areconnected to inputs of the controller 25 via measuring lines 28. Twooutputs of the controller 25 are connected to the rotor slip rings 22,23 via control lines 29. Like the control unit 18, the controller 25 isattached to the connecting terminals 30, 31 of the direct-current driveand therefore to the DC voltage network. Directly after the connectingterminal 30, there is a circuit breaker 32, which can be triggered bythe controller 25.

[0019] The controller 25 measures the amount and phase of the currentsflowing through the instrument shunts 26 and adds these vectorially.When the direct-current drive is functioning properly, the result ofthis addition is always zero. If the vector sum deviates significantlyfrom zero, then there is a malfunction in the winding phases 151-153 orin the power switches 17. In this case, the controller 25 triggers thecircuit breaker 32 so that it opens and applies an excitation current tothe field excitation winding 21, which current, by means of the coils24, generates a magnetic flux oriented in the opposite direction fromthe magnetic flux of the permanent magnet poles 20. This weakens theexcitation field of the synchronous motor 10 so that no voltage or onlya low voltage is induced in the stator winding 15 and therefore nobraking moment or only a slight braking moment occurs. The magneticfield is weakened as a function of the temperature of the synchronousmotor 10, which is detected by the temperature sensor 27 and supplied tothe controller 25. The temperature-dependent weakening of the magneticfield occurs in such a way that an irreversible demagnetization of thepermanent magnet poles 20 is avoided and therefore permanent damage tothe synchronous motor 10 is prevented. The magnitude of the excitationcurrent applied to the rotor slip rings 22, 23 by the controller 25,which current can be adjusted by means of pulse-to-width modulation, iscontrolled as a function of the strand current in the stator winding 15,i.e. the excitation current is increased until the field weakening thusachieved induces only a low voltage and therefore only a lowshort-circuit current flows, which generates a still acceptable brakingmoment. If this reduction of the braking moment is insufficient, thenthe resulting excitation field can be brought to zero, but permanentdamage to the permanent magnet poles 20 must then be accepted.

[0020] The device for producing a fail-silent behavior canadvantageously also be used during normal operation of thedirect-current drive. If the current direction in the field excitationwinding 21 is selected so that the magnetic field generated by it isadded to the magnetic field of the permanent magnet poles 20, then thisresults in a motor with a high power density. If the current directionin the field excitation winding 21 is reversed and therefore theresulting magnetic field is weakened, then higher speeds can be set. Themagnitude of the field strengthening or field weakening is in turn setby the controller 25 by means of pulse-to-width modulation of the DCcurrent supplied to the rotor slip rings 22, 23. The circuit breaker 32is not triggered.

1. A brushless direct-current drive with a synchronous motor (10), whichhas a stator (12) that supports a multi-phase stator winding (15) and arotor (13) with permanent magnet poles (20), which generate a magneticflux that penetrates the stator winding (15), and with a switch unit(11), which precedes the stator winding (15) and is controlled by anelectronic control unit (18), for commutating the stator winding (15),characterized in that the rotor (13) contains a field excitation winding(21), which can be supplied with current in the event of a malfunctionso that it generates a magnetic flux oriented in the opposite directionfrom the magnetic flux of the permanent magnet poles (20).
 2. Thedirect-current drive according to claim 1, characterized in that thefield excitation winding (21) has a number of coils (24) thatcorresponds to the number of permanent magnet poles (20), each of whichcoils (24) is wound around one of the permanent magnet poles (20). 3.The direct-current drive according to claim 2, characterized in that thecoils (24) are connected in parallel or in series and are connected to apair of rotor slip rings (22, 23) that are connected to the rotor (13)in a non-rotating fashion.
 4. The direct-current drive according toclaim 3, characterized in that the rotor slip rings (22, 23) can beacted on with a direct current whose magnitude can be set by means of acontroller (25).
 5. The direct-current drive according to claim 4,characterized in that the controller (25) is integrated into theelectronic control unit (18).
 6. The direct-current drive according toclaim 4 or 5, characterized in that the controller (25) has apulse-to-width modulator.
 7. The direct-current drive according to oneof claims 4-6, characterized in that the controller (25) adjusts the DCvoltage as a function of the motor temperature.
 8. The direct-currentdrive according to one of claims 4-7, characterized in that thecontroller (25) adjusts the DC voltage as a function of a strand currentflowing in the stator winding (15).
 9. A brushless direct-current drivewith a synchronous motor (10), which has a stator (12) that supports amulti-phase stator winding (15) and a rotor (13) with permanent magnetpoles (20), which generate a magnetic flux that penetrates the statorwinding (15), and with a switch unit (11), which precedes the statorwinding (15) and is controlled by an electronic control unit (18), forcommutating the stator winding (15), characterized in that the rotor(13) contains a field excitation winding (21), which has a number ofcoils (24) that corresponds to the number of permanent magnet poles(20), each of which coils (24) is wound around one of the permanentmagnet poles (20).
 10. The direct-current drive according to claim 9,characterized in that the coils (24) are connected in parallel or inseries and are connected to rotor slip rings (22, 23).
 11. Thedirect-current drive according to claim 9 or 10, characterized in thatthe coils (24) are connected so that the directions of the magneticfluxes generated in coils (24) that succeed one another in the rotationdirection are oriented in opposite directions.
 12. The direct-currentdrive according to claim 11, characterized in that the rotor slip rings(22, 23) can be acted on with a DC voltage whose magnitude can beadjusted by means of a controller (25), preferably by means of apulse-to-width modulation.